1. Field of the Invention
This invention relates to an electronic sensor, and, in particular, to a sensor suitable for detecting the location of studs behind a variety of surfaces, including walls, floors and similar type structures. More specifically, the invention relates to an electronic stud sensor with two sensitivity modes for determining the location of studs behind either thick or thin surfaces, and with the ability to inform the user when the sensor has been calibrated over a stud.
2. Description of the Prior Art
U.S. Pat. No. 4,464,622 issued Aug. 7, 1984, and incorporated by reference herein, discloses an electronic wall stud sensor particularly suitable for locating a wall stud positioned behind a wall surface. The sensor detects the stud by measuring a change in the capacitance of the wall due to the presence of a stud while the sensor is moved along the wall surface. The sensor includes a plurality of capacitor plates mounted in the sensor close to the wall surface, a circuit for detecting any changes in the capacitance of the capacitor plates due to a change in the dielectric constant of the wall caused by the location of a stud positioned behind the wall surface and immediately adjacent to the capacitor, and an indicator for indicating the change in capacitance of the capacitor plate, thereby indicating the wall stud position. The sensor also alerts the operator when calibration is occurring.
While the above described electronic wall stud sensor operates as described to locate studs in walls, experience has shown that its performance could be improved in several respects. The stud sensor is unable to reliably sense studs through surfaces significantly thicker than the typical 5/8 inch sheetrock wall. For instance, the sensor can not sense floor joists (another type of stud) under a combination of 3/4 inch subfloor and 1/4 inch oak flooring.
Moreover, the stud sensor is incapable of informing the operator when the sensor has been calibrated over a stud. When the sensor is subsequently moved from the stud after completion of this incorrect calibration, it is unable to detect studs and no indication is given that the sensor was incorrectly calibrated. Finally, the stud sensor's correct calibration voltage has a tendency to "leak off," due to use of a sample-and-hold capacitor. This results in less accurate readings after approximately a minute of use.
Therefore, there is a need to sense studs through both thick and thin surfaces. There is also a need to determine when the device has been calibrated over a stud, and informing the operator of this. Further, there is a need for a circuit which maintains calibration indefinitely.